In acoustic welding, such as ultrasonic welding, two parts to be joined (typically thermoplastic parts) are placed directly below a vibrational element, typically an ultrasonic horn. In plunge welding, the horn plunges (travels towards the parts) and transmits vibrations into the top part. Typically these vibrations are ultrasonic. The vibrations travel through the top part to the interface of the two parts. At that interface, the vibrational energy is converted to heat due to intermolecular friction that melts and fuses the two parts. When the vibrations stop, the two parts solidify under force, producing a weld at the joining surface.
Continuous ultrasonic welding is typically used for sealing fabrics, films, and other parts. In continuous welding, the ultrasonic horn is typically stationary and the part is moved beneath it. Scan welding is a type of continuous welding in which the part moves. The plastic part is scanned beneath one or more stationary horns. In transverse welding, the part is stationary while the horn moves over it.
The horn is an acoustical tool made of, for example, aluminum, titanium, or sintered steel that transfers the mechanical vibratory energy to the part. Horn displacement or amplitude is the peak-to-peak movement of the horn face. The ratio of horn output amplitude to the horn input amplitude is termed gain. Gain is a function of the ratio of the mass of the horn at the vibration input and output sections. Generally, in horns, the direction of amplitude at the welding surface of the horn is coincident with the direction of the applied mechanical vibrations.
A rotary acoustic horn, like all horns, imparts energy at a selected wavelength, frequency, and amplitude. The rotary horn includes a shaft with input and output ends, and a welding portion mounted on and coaxial with the output end. The diameter of the welding portion is typically greater than the diameter of the shaft. The welding portion has a cylindrical weld face having a diameter that expands and contracts with the application of vibration energy. Typically, a rotary horn is cylindrical and rotates about a longitudinal axis. The input vibration is in the axial direction and the output vibration is in the radial direction. The horn and anvil are close to each other, and the anvil can rotate in the opposite direction of the horn. The part to be bonded passes between these cylindrical surfaces at a linear velocity, which equals the tangential velocity of the cylindrical surfaces. Matching the tangential velocities of the horn and the anvil with the linear velocity of the material is intended to minimize the drag between the horn and the material. The excitation in the axial direction is similar to that in conventional plunge welding.
There are typically two methods of mounting an ultrasonic horn, nodal mounting and non-nodal mounting. A node is a position of the horn that is not moving in one or more directions. With a nodal mount the horn can be held or grasped rigidly. Non-nodal mounts require some flexible elements because the horn surface is moving (vibrating). The nodes on a horn typically extend circumferentially about the horn, and are spaced axially along the length of the horn.
Anti-nodes are areas of maximum displacement of the horn or booster, attaching mounting systems at these locations or at other non-nodal locations requires the mount to be designed to isolate the vibration from the horn.
U.S. Pat. No. 3,955,740 discloses a non-nodal rotary horn mount, which uses a solid metal diaphragm located in the junction between the booster and the horn. Static loads can be borne because the design uses a rotating tube to isolate this force from the bearings. Also, in this design the diaphragm is designed to resonate at the frequency of the horn. The diaphragm is an ultrasonic element. Another type of nodal mount utilizes a series of setscrews positioned radially around the node. To position the mount about the horn, the setscrews are threaded through the nodal mount and contact the surface of the horn. Typically they extend into a groove or notch in the surface of the horn to assure the mount maintains its position relative to the horn. While the setscrew type mount can isolate vibrations from the vibrating element, the static load, which can be imparted onto the mount, is limited to the ability of the setscrews to withstand the force.